The present invention pertains to a method for improving the quality of seismic data and more particlarly to the migration of seismic common depth point gathers.
Seismic data is gathered by generating acoustic pulses into the earth's surface ("shooting") and detecting the reflected or refracted waves with acoustic pulse detectors such as geophones. When acoustic pulses come in contact with an interface, such as an oil/gas, oil/water or shaley sand/salt dome, a portion of the acoustic wave will be reflected back to the surface. Several "shot" points are commonly used in a row in conjunction with a line of detectors or geophones. The line of shot points together with the line of geophones essentially define a slice or plane of the earth perpendicular to the earth's surface.
In theory, the detected acoustic pulses yield a map of this plane indicating interfaces between substances having different acoustical properties. Unfortunately, this theory is applicable only as long as the interfaces are parallel to the surface of the earth. Interfaces are not always parallel to the earth's surface and a change in acoustical properties may be misplaced on the map of the subsurface plane. For this reason, a process of migration is routinely performed.
Migration is a process which maps seismic wavefields which are recorded in the time domain into a depth domain through a wave equation and a suitable velocity field. Normally, the data is processed before migration. A velocity is determined and the data gathered by the acoustic pulse detectors is stacked; data gathered at a common depth point from several detectors is combined by methods known in the art. The velocity field is normally determined from the velocity used to stack the seismic data. This determination is based on the fact that migration methods presently in use are insensitive to reasonable velocity errors. Newer migration methods allow lateral velocity variation but are extremely sensitive to velocity errors and are used infrequently and cautiously.
Additional methods of migration have been developed which migrate the detected data before stacking. While prestack migration is more desirable than poststack migration, since stacking tends to destroy diffractions, buried foci and other wave phenomena present in unstacked data, no method of prestack migration had previously been developed which is economically feasible for routine seismic processing.
Present methods for migration all have a common deficiency, a velocity field must be picked from the seismic data or nearby well logs and used as an input to the migration process.